Stamen (Microsporophyll) – It is the male reproductive unit of angiosperms.
It consists of two parts:-
- Filament – a long and slender stalk and its proximal end is attached to the thalamus or petal of the flower.
- Anther – The anther is bilobed structure and each lobe having two theca (i.e. dithecous), so there is two theca in each lobe thus anther is tetragonal structure.
- The anther consists of four microsporangia located at the corner, two in each lobe.Microsporangia develop further into pollen sacs.
Structure of Microsporangia
- The microsporangium consists of 4-5 wall layers of cells – Epidermis, Endodermis, Middle layer and Tapetum.
- Epidermis, Endodermis, Middle layer – Perform the function of protection and help in dehiscence of the anther.
- Tapetum – It is the innermost layer and has more than one nucleus. It nourishes the developing pollen grain.
- when the anther is young, the center of each microsporangium occupied by sporogenous tissues ( a group of compactly arranged tissues), thus a typical anther is tetrasporangiate.
- The formation and differentiation of microspore (Pollen grain) from Pollen Mother cell (PMC) is called Microsporogenesis.
As anther develop,
- The cells of sporogenous tissue ( in the center of microsporangium) undergo mitosis and form microspore tetrad. Each one is a potential Pollen or a microspore mother cell.
As the anther mature,
- It dehydrates and the microspore tetrad dissociates from each other and develops into pollen grains.
Pollen grain (Microspore)
- The pollen grain represents the male gametophyte.
- The pollen grain develops inside the pollen sac while developing they consume the middle layer and tapetum; such that mature anther has 2 layers – epidermis and endothecium.
- Shape and size of pollen grain – Generally spherical and 25 – 50 µm.
- Pollen grain has a prominent two layered wall:-
- Exine – outer
- Intine – inner
- It is made up of sporopollenin.
- Sporopollenin can withstand high temperatures, strong acid, and alkali. It is the most resistant organic material know.
- No sporopollenin degrading enzyme is known.
Germ pore – It is the prominent aperture on exine layer where sporopollenin is absent.
- It is thin an continuous layer made of cellulose and pectin.
- It emerges out as the pollen tube from the germ pore during germination.
As already discussed, The outer surface of microspores may have spines, ridges or furrows which may vary in other ways in different species. There may be oval, ellipsoidal, triangular, pollen grains. A two layered wall covers the cytoplasm. The outer layer exine covered by sporopollenin. Sporopollenin is resistant to chemical and biological decomposition. In insect-pollinated pollen, the exine is covered by a yellowish, viscous and sticky substance called pollen kit.
Development of Male gametophyte
When pollen grain matures it contains two cells:-
- Vegetative cells or tube cell
- Generative cell
- The vegetative cell is bigger and has sufficient food reserve with a large irregularly shaped nucleus.
- They are small and floats in the cytoplasm of the vegetative cell.
- It is spindle shaped with dense cytoplasm and a nucleus.
- In a large majority of flowering plants, pollens are shed at the two-celled stage.
- In remaining species, the generative divide mitotically to give rise to the two male gametes before pollen grain are shed (3-cell stage).
- The pollen of many species cause severe allergies and bronchial affliction and often reach to chronic respiratory disorders – Asthma, Bronchitis etc.
Example: – Parthenium or Carrot grass